This invention relates to a variable venturi carburetor for an internal combustion engine which controls the air-fuel ratio by air bleed control operation.
In a conventional carburetor, the air-fuel ratio is controlled by varying the amount of bleed air admitted into an air chamber in the fuel jet of the carburetor. Such an air chamber is the annular space defined between the first orifice formed by the fuel metering portion of the fuel jet and by the fuel metering needle and the second orifice formed by the fuel-air mixture discharge portion of the fuel jet and by the fuel metering needle. The amount of bleed air is controlled by the actuator which is operated in response to the output signal transmitted from a computer into which the signals transmitted from sensors for sensing the engine running conditions are inputted. In such a control system, the fuel injected from the fuel metering portion and the bleed air delivered through an air bleed passage are mixed in the said annular space and the fuel-air mixture is discharged into the venturi portion. The flow rate of the fuel or the bleed air and the relative flow rate thereof are varied with the engine running conditions. In general, there arises a case where the flow rate is unstable with respect to a certain value of gas or liquid flow rate and relative flow rate thereof. Under these conditions, when the fuel-air mixture is metered by the said second orifice, the mixture is disadvantageously variably discharged. To avoid this drawback, the inside diameter of the discharge portion may be enlarged so as for the fuel-air mixture not to variably discharged. However, in this case, there arises a problem that "bleed sensitivity", that is, the variation in the air-fuel ratio relative to the variation in the restriction created by the actuator, is reduced. This is caused by the fact that the pressure variation within the annular space relative to the constant variation in the restriction created by the actuator is decreased with an increase in the annular opening area of the second orifice formed at the discharge portion.
In another conventional variable venturi carburetor including a fuel jet provided with a jet portion, the jet portion is provided with an air bleed aperture through which bleed air is delivered into the annular restriction opening defined between the fuel metering needle and the jet portion. In this case, the amount of bleed air may be controlled by the sensors, computer and actuator as mentioned in connection with the preceding variable venturi carburetor, thereby permitting the air-fuel ratio to be controlled. As the amount of induced air flowing through the venturi portion of the air intake passageway is decreased, the said annular restriction opening area at the jet portion is decreased, and therewith the "bleed sensitivity" is increased. When the amount of induced air is further decreased until the engine reaches the idle condition, the air bleed aperture into the jet portion is nearly closed by the fuel metering needle (see FIG. 6). Accordingly, it should be apparent that the air-fuel ratio cannot be controlled by the air bleeding operation as depicted by the solid line in FIG. 7.